Controlling static charge is an important issue in continuous web operations (product moved in a continuous or nearly continuous feed) and in semiconductor manufacturing. Undesirable Triboelectric (static caused by friction) charges are introduced onto the web during handling by rollers, cutters and the like. In web operations, such undesirable charges can attract unwanted particulate matter onto the product, can cause difficult handling issues with the product, and may even cause discharges which are potentially harmful to the electronic controls that operate the machines. In semiconductor manufacturing, device defects caused by electrostatically attracted foreign matter and electrostatic discharge events contribute greatly to overall manufacturing losses.
Air ionization is an effective method of eliminating static charges on non-conductive materials and isolated conductors. Air ionizers generate large quantities of positive and negative ions in the surrounding atmosphere which serve as mobile carriers of charge in the air. As ions flow through the air, they are attracted to oppositely charged particles and surfaces. Neutralization of electrostatically charged surfaces can be rapidly achieved through this process.
Air ionization may be performed using electrical ionizers which generate ions in a process known as corona discharge. Electrical ionizers generate air ions through this process by intensifying an electric field around a sharp point until it overcomes the dielectric strength of the surrounding air. Negative corona occurs when electrons are flowing from the electrode into the surrounding air. Positive corona occurs as a result of the flow of electrons from the air molecules into the electrode.
Ionizer devices take many forms such as ionizing bars, air ionization blowers, air ionization nozzles, and the like, and are utilized to neutralize static electrical charge by emitting positive and negative ions into the workspace or onto the surface of an area carrying undesirable static charges. Ionizing bars are typically used in continuous web operations such as paper printing, polymeric sheet material, or plastic bag fabrication. Air ionization blower and nozzles are typically used in workspaces for assembling electronics equipment such as hard disk drives, integrated circuits, and the like, that are sensitive to electrostatic discharge (ESD).
To achieve the maximum possible reduction in static charges from an ionizer of a given output, the ionizer must produce amounts of positive and negative ions in order to compensate for the net charge on the web or in the workspace. That is, the output of the ionizer must increase or decrease the output of positive and/or negative ions in order to achieve a neutralized net charge on the web or in the workspace.
One prior art method of generating ions is by use of an alternating current (AC) voltage generator connected to ionizing pins which produces ions of one polarity for approximately 35% of a half cycle and then, after a delay, produces ions of the other polarity for approximately 35% of a half cycle. The positive ions and negative ions are output based upon the cycle or frequency of the AC voltage waveform and are not controlled based upon feedback of the actual charge on the web or in the workspace or on the demand for ions of a particular polarity. Such prior art devices are discussed in U.S. Pat. No. 3,936,698 (Meyer) and U.S. Pat. No. 3,714,531 (Takahashi). The drawback to AC ionizers is that when the net charge is negative on the web or in the workspace is negative, negative ions are still going to be generated and possibly directed onto the web or into the workspace, which will have no effect in neutralizing the negative charge. The converse is also true when the net charge is positive. Additionally, there is a lag time between generating positive ions and negative ions directly related to the rise time of the AC voltage waveform. Thus, ions are produced in slugs rather than in a continuous manner which further is not effective in neutralizing a charge in applications with a fast moving web.
Another prior art method for generating ions is by use of a high voltage direct (DC) current generator of each polarity connected to ionizing pins. Some of these DC generators are merely fixed output power supplies that generate a continuos output of both positive and negative ions. A user may take readings with a handheld charge monitor and then make adjustments to the positive or negative power supply accordingly. However, the change in charge on the web or the workspace can occur very quickly and very frequently. Simple changes to the ambient conditions such as temperature, humidity, and the like, can have a drastic affect on the Triboelectric charging that the materials being handled experience. Thus, it is not possible for a user to make adjustments often enough (continuously) to compensate for the charge fluctuations.
One prior art device described in U.S. Pat. No. 5,930,105 describes a control circuit for a DC ionizer that attempts to trim the outputs of the positive and negative power supplies by monitoring the total net current through a resistor that purportedly corresponds to a change in the charge value. The single return current sensing resistor compensates the outputs of both power supplies, and does not separately control them. Thus, there is always a given output level of each type of ion, positive and negative, which does not compensate for charge changes quickly enough, especially in a fast-moving web. This DC ionizer only produces slightly more of one ion over the other and therefore the net charge that reaches the web or workspace has little affect when the charge on the web or workspace changes quickly.
Accordingly, there is an unmet need for a controller for a DC ionization system which allows for fast response time and achieves improved charge neutralization. The present invention fulfills these needs.
The present invention provides a control circuit for an ionizer which controls an output of at least one of a positive voltage direct current power supply and a negative voltage direct current power supply. Each power supply is connected to at least one ionizing pin. The control circuit controls the output of at least one of the power supplies so as to cause a flow of positive and negative ions to be emitted from the ionizer and directed towards a target. The control circuit includes a positive power supply return current sense resistor that biases the positive voltage power supply to increase the output of the positive voltage power supply when a positive ion current detected by the positive power supply return current sense resistor decreases and to decrease the output of the positive voltage power supply when the positive ion current detected by the positive power supply return current sense resistor increases, thereby creating a static-free environment at the target. The control circuit also includes a negative power supply return current sense resistor that biases the negative voltage power supply to decrease the output of the negative voltage power supply when a negative ion current detected by the negative power supply return current sense resistor increases and to decrease the output of the negative voltage power supply when the negative ion current detected by the negative power supply return current sense resistor increases, thereby creating the static-free environment at the target.
In an alternate embodiment, the positive power supply return current sense resistor biases the negative voltage power supply to decrease the output of the negative voltage power supply when a positive ion current detected by the positive power supply return current sense resistor increases and to increase the output of the negative voltage power supply when the positive ion current detected by the positive power supply return current sense resistor decreases, thereby creating the static-free environment at the target. The negative power supply return current sense resistor biases the positive voltage power supply to decrease the output of the positive voltage power supply when a negative ion current detected by the negative power supply return current sense resistor increases and to increase the output of the positive voltage power supply when the negative ion current detected by the negative power supply return current sense resistor decreases, thereby creating the static-free environment at the target.